Positive type photoresist compositions are normally compositions comprising an alkali-soluble resin and a naphthoquinone diazide compound as a light-sensitive material. Examples of such compositions include novolak type phenol resin/naphthoquinone diazide-substituted compounds as disclosed in U.S. Pat. Nos. 3,666,473, 4,115,128, and 4,173,470. Most typical examples of such compositions include a novolak resin made of cresol formaldehyde/trihydroxybenzophenone-1,2-naphthoquinonediazidosulfonic ester, as disclosed in L. F. Thompson, "Introduction to Microlithography", ACS, No. 219, pp. 112-121.
Novolak resin, a binder, can be dissolved in an alkaline aqueous solution without swelling. The novolak resin can also exhibit a high resistance particularly to plasma etching when an image thus produced is used as a mask for etching. Thus, novolak resin is particularly useful in this application. As a light-sensitive material, a naphthoquinone diazide compound serves as a dissolution inhibitor for reducing the alkali solubility of novolak resin. But it is peculiar in that it undergoes decomposition upon irradiation with light to produce an alkali-soluble substance which rather enhances the alkali solubility of novolak resin. Because of the great change in properties resulting from irradiation with light, a naphthoquinone diazide compound is particularly useful as a light-sensitive material for positive type photoresist.
From this standpoint, many positive type photoresists comprising novolak resin and naphthoquinone diazide light-sensitive material have heretofore been developed and put to practical use. These positive type photoresists have attained sufficient results in working lines of a width of 1.5 to 2 .mu.m.
However, integrated circuits now have greater degree of integration. It is a recent tendency that the working of ultrafine patterns formed of lines of a width of 1 .mu.m or less is required in the production of semiconducting substrates such as SLSI. In such applications, a photoresist having a high resolution, a high accuracy of reproduction of exposure mask pattern and a high sensitivity for high productivity has been desired.
In order to add to the degree of integration in integrated circuits, the etching process has been switched from a conventional wet etching process to a dry etching process. Since the dry etching process causes the temperature of the resist to be elevated, the resist must exhibit a high heat resistance to thermal deformation.
As approaches for improving the heat resistance of the resist, there have been proposed the use of a resin free of a component having a weight-average molecular weight of 2,000 or less as disclosed in JP-A-60-97347 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and the use of a resin wherein the sum of the content of monomer to trimer is 10% by weight or less as disclosed in JP-A-60-189739.
However, the use of the above mentioned resin free of or having a reduced low molecular component content is disadvantageous in that it normally reduces sensitivity, reducing the throughput in the production of devices.
Some have tried to improve the sensitivity and developability of the resist by blending a specific compound in the resist composition. Thus, JP-A-61-141441 discloses a positive type photoresist composition containing trihydroxybenzophenone. This trihydroxybenzophenone-containing positive type photoresist exhibits an improved sensitivity and developability but shows a deteriorated heat resistance due to the addition of trihydroxybenzophenone.
JP-A-64-44439, JP-A-1-177032, JP-A-1-280748, and JP-A-2-10350 disclose an approach for providing a high sensitivity without deteriorating the heat resistance by incorporating an aromatic polyhydroxy compound other than trihydroxybenzophenone in the system. However, this approach does not necessarily provide sufficient improvement in the developability of the system.